Why do girls ask do I know you?

So in the land of online dating it's not unusual for me to message cute girls on facebook. Occassionally if I comment on something, this will be the stuck up response I get. Usually it's not a good thing if they've said this i've found. It's not like I message anything offensive to them, so I always wonder why I get this as a response.

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Turned out she was married. Took her all of two seconds to mention that one. So I responded saying, oh so you're just automatically assuming I like you already? lmao.
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fyi: It didn't show she was married, otherwise I wouldn't have bothered. I'm not a total idiot.
Why do girls ask do I know you?
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